1. Field of the Invention
The present invention relates to a longitudinal-mode surface acoustic wave filter, a method of manufacturing a surface acoustic wave filter, and a communication device.
2. Related Art of the Invention
In recent years, surface acoustic wave filters have been widely used in mobile communication devices. Surface acoustic wave filters of a longitudinal mode type or a ladder type are used as a filter in a radio frequency (RF) stage. With the improvement in performance of communication devices such as portable telephones, there has been an increasing demand for reducing the loss and increasing the attenuation in surface acoustic wave filters.
A conventional longitudinal-mode surface acoustic wave filter will be described.
FIG. 12 shows a configuration of a conventional longitudinal-mode surface acoustic wave filter. As shown in FIG. 12, the surface acoustic wave filter has a piezoelectric substrate 801, first, second, and third interdigital transducer (IDT) electrodes 802, 803, and 804, and first and second reflector electrodes 805 and 806, the IDT electrodes and the reflector electrodes being formed on the substrate. The upper electrode fingers of each of the second and third IDT electrodes 803 and 804 is connected to an input terminal IN, while the lower electrode fingers of each of the second and third IDT electrodes 803 and 804 is grounded. The lower electrode fingers of the first IDT electrode 802 is connected to an output terminal OUT, while the upper electrode fingers of the first IDT electrode 802 is grounded. The distances between centers of adjacent pairs of the electrode fingers of the first, second, and third IDT electrodes 802, 803, and 804, represented by the distance indicated by P in FIG. 12 (hereinafter referred to as xe2x80x9cpitchxe2x80x9d), are equal to each other. The longitudinal-mode surface acoustic wave filter is thus constructed.
In the above-described surface acoustic wave filter, the electrode fingers are arranged with a constant pitch in order that the acoustic velocity of a surface acoustic wave be constant through the arrangement of the first, second, and third IDT electrodes 802, 803, and 804. In many instances, however, the number of electrode fingers of the first IDT electrode 802 and that of each of the second and third IDT electrodes 803 and 804 are set different from each other according to a design considering the bandwidth and impedance. Ordinarily, the surface acoustic wave filter is designed so that the number of electrode fingers of the first IDT electrode 802 is larger than that of each of the second and third IDT electrodes 803 and 804.
A longitudinal-mode surface acoustic wave filter has also been used which is designed so that the electrode fingers of each of electrodes have different pitches as shown in FIG. 13 to achieve a reduction in loss for example. The conventional longitudinal-mode surface acoustic wave filter shown in FIG. 13 is based on a design in which each IDT electrodes have an electrode finger pitch different from that in a main region.
Referring to FIG. 13, the surface acoustic wave filter has a piezoelectric substrate 1201, first, second, and third IDT electrodes 1202, 1203, and 1204, and first and second reflector electrodes 1205 and 1206, the IDT electrodes and the reflector electrodes being formed on the substrate. The upper electrode fingers of each of the second and third IDT electrodes 1203 and 1204 is connected to an input terminal IN, while the lower one of each of the second and third IDT electrodes 1203 and 1204 is grounded. The lower electrode fingers of the first IDT electrode 1202 is connected to an output terminal OUT, while the upper electrode fingers of the first IDT electrode 1202 is grounded.
Also, referring to FIG. 13, if the pitch in a region indicated by 1a in the first IDT electrode 1202 is P, P is xc2xd wavelength. If the pitch in a region indicated by 1b is Pxe2x80x2, Pxe2x80x2 is smaller than xc2xd wavelength. The pitch in a region indicated by 2a in the second IDT electrode 1203 is P, and P is xc2xd wavelength. The pitch in a region indicated by 2b is Pxe2x80x2, and Pxe2x80x2 is smaller than xc2xd wavelength. Similarly, the pitch in a region indicated by 3a in the third IDT electrode 1204 is P, and P is xc2xd wavelength. The pitch in a region indicated by 3b is Pxe2x80x2, and Pxe2x80x2 is smaller than xc2xd wavelength.
Thus, in each of the first IDT electrode 1202, the second IDT electrode 1203, and the third IDT electrode 1204, different electrode finger pitches are set between the electrode fingers in the same IDT electrodes.
Also in many instances relating to the arrangement shown in FIG. 13, the number of electrode fingers of the first IDT electrodes 1202 and that of each of the second and third IDT electrodes 1203 and 1204 are set different from each other according to a design considering the bandwidth and impedance. Ordinarily, the surface acoustic wave filter is designed so that the number of electrode fingers of the first IDT electrode 1202 is larger than that of each of the second and third IDT electrodes 1203 and 1204.
There is a problem in such a surface acoustic wave filter that there is a limit to improvement in filter characteristics in achieving a wide-band characteristic.
In view of the above-described problem, an object of the present invention is to provide a wide-band surface acoustic wave filter having a steep out of-band attenuation characteristic, a method of manufacturing the surface acoustic wave filter, and a communication device.
One aspect of the present invention is a surface acoustic wave filter comprising:
a piezoelectric substrate;
at least an input IDT electrode arranged on said piezoelectric substrate; and
at least an output IDT electrode arranged on said piezoelectric substrate,
wherein a pitch of electrode fingers of said input IDT electrode and a pitch of electrode fingers of said output IDT electrode are different from each other.
Another aspect of the present invention is the surface acoustic wave filter, wherein the pitch of electrode fingers of the IDT electrode larger in number of electrode fingers in said input and output IDT electrode is larger than the pitch of electrode fingers smaller in number of electrode fingers.
Still another aspect of the present invention is a surface acoustic wave filter comprising:
a piezoelectric substrate;
at least an input IDT electrode arranged on said piezoelectric substrate; and
at least an output IDT electrode arranged on said piezoelectric substrate,
wherein the metalization ratio of said input IDT electrodes and the metalization ratio of said output IDT electrodes are different from each other.
Yet still another aspect of the present invention is the surface acoustic wave filter, wherein the metalization ratio of an IDT electrode larger in number of electrode fingers in said input and output IDT electrodes is lower than the metalization ratio of an IDT electrode smaller in number of electrode fingers.
Still yet another aspect of the present invention is the surface acoustic wave filter, wherein if an IDT electrode has a plurality of electrode finger pitches, the pitch of main excitation electrode fingers is set as a basic pitch.
A further aspect of the present invention is the surface acoustic wave filter, wherein a peak frequency of a radiation characteristic of said input IDT electrode is substantially equal to a peak frequency of a radiation characteristic of said output IDT electrode.
A still further aspect of the present invention is the surface acoustic wave filter, wherein one of said input IDT electrode and said output IDT electrode comprises a first IDT electrode including a pair of electrode fingers opposed to each other;
the other of said input IDT electrode and said output IDT electrode comprises a second IDT electrode including a pair of electrode fingers opposed to each other, and a third IDT electrode including a pair of electrode fingers opposed to each other, said second IDT electrode being placed on one side of said first IDT electrode, said third IDT electrode being placed on the other side of said first IDT electrode;
said first, second, and third IDT electrodes are arranged along a direction in which a surface acoustic wave propagates; and
the peak frequency of the radiation characteristic of said first IDT electrode is substantially equal to the peak frequency of the radiation characteristic of each of the second and third IDT electrodes.
A yet further aspect of the present invention is the surface acoustic wave filter, wherein one of said input IDT electrode and said output IDT electrode comprises first, fourth, and fifth IDT electrodes each including a pair of electrode fingers opposed to each other;
the other of said input IDT electrode and said output IDT electrodes comprises a second and third IDT electrodes each including a pair of electrode fingers opposed to each other;
said second and third IDT electrodes are placed on opposite sides of said first IDT electrode;
said fourth IDT electrode are placed on the side of said second IDT electrodes opposite from the side on which said first IDT electrode are placed;
said fifth IDT electrode are placed on the side of said third IDT electrode opposite from the side on which said first IDT electrode are placed;
said first, second, third, forth and fifth IDT electrodes are arranged along a direction in which a surface acoustic wave propagates; and
the peak frequencies of the radiation characteristics of at least more than one of the group of said first IDT electrode, and the group of said fourth and fifth IDT electrodes, and the group of said second and third IDT electrodes are substantially equal to each other.
A still yet further aspect of the present invention is the surface acoustic wave filter, wherein the film thickness of said first IDT electrode and the film thickness of each of said second and third IDT electrodes are different from each other.
An additional aspect of the present invention is the surface acoustic wave filter, wherein the material of said first IDT electrode and the material of each of said second and third IDT electrodes are different from each other.
A still additional aspect of the present invention is the surface acoustic wave filter, wherein the metalization ratio of said first IDT electrode and the metalization ratio of each of said second and third IDT electrodes are equal to each other;
the number of electrode fingers of said first IDT electrode is larger than the number of electrode fingers of each of said second and third IDT electrodes; and
the electrode finger pitch of said first IDT electrode is larger than the electrode finger pitch of each of said second and third IDT electrodes.
A yet additional aspect of the present invention is the surface acoustic wave filter, wherein the metalization ratio of said first IDT electrode, the metalization ratio of said second IDT electrode and the metalization ratio of said third IDT electrode are different from each other.
A still yet additional aspect of the present invention is the surface acoustic wave filter, wherein a plurality of filter tracks each having first, second, and third IDT electrodes, and first and second reflector electrodes are formed on said piezoelectric substrate, and said plurality of filter tracks function as one filter in cooperation with each other.
A supplementary aspect of the present invention is the surface acoustic wave filter, wherein each of said plurality of filter tracks is identical in configuration to the others.
A still supplementary aspect of the present invention is the surface acoustic wave filter, wherein at least one of said plurality of filter tracks is different in configuration from the others.
A yet supplementary aspect of the present invention is the surface acoustic wave filter, further comprising a first reflector electrode placed on the opposite side of said second IDT electrode on said piezoelectric substrate opposite from the side on which said first IDT electrode are placed; and
a second reflector electrode placed on the side of said third IDT electrode on said piezoelectric substrate opposite from the side on which said first IDT electrode are placed,
wherein said first, second, and third IDT electrodes and said first and second reflector electrodes are arranged along a direction in which a surface acoustic wave propagates.
A still yet supplementary aspect of the present invention is a method of manufacturing a surface acoustic wave filter, comprising
a piezoelectric substrate;
an input IDT electrode arranged on the piezoelectric substrate; and
an output IDT electrode arranged on the piezoelectric substrate,
wherein said method makes a pitch of electrode fingers of said input IDT electrode and a pitch of electrode fingers of said output IDT electrode different values.
Another aspect of the present invention is a communication device comprising:
a transmitting circuit which outputs a transmitted wave; and
a receiving circuit to which a wave to be received is input,
wherein a surface acoustic wave filter is used in said transmitting circuit and/or in said receiving circuit.
Still another aspect of the present invention is a communication device comprising:
a transmitting circuit which outputs a transmitted wave; and
a receiving circuit to which a wave to be received is input,
wherein the surface acoustic wave filter is used in said transmitting circuit and/or in said receiving circuit.